Warehouse workers or the like regularly employ stepped structures such as mobile ladders, scaffolds and platforms to reach a particular work location. In the course of completing their tasks, it is common practice for a worker to carry various tools, supplies, accessories and other articles while climbing the stepped structure. For example, a retail worker may scale a ladder using one or both hands to transport inventory to a desired elevation. Such practice not only jeopardizes the safety of the worker, but may require multiple trips up and down the ladder which contributes to the inefficiency of the task.
In an effort to remedy these problems, the assignee of the present invention has previously produced a mobile ladder that is provided with a motorized lifting mechanism for selectively assisting the worker in raising and lifting a loaded tray thereby enabling the worker to use both hands to grasp handrails while climbing. This prior art design (See FIG. 13) includes a metal lifting tray that is slidably mounted upon a track assembly and connected to a cable which is engaged about a pulley at the top of the track assembly. The cable extends downwardly along the front side of the track assembly and is wound around a lower pulley fixed on a mounting plate on a movable base of the ladder. The free end of the cable is attached to a winch, which has a motor driven by a battery. Both the winch and battery are secured on the mounting plate.
While this combined ladder and lifting mechanism has generally been useful, it has been found that the winch motor is extremely noisy and also creates an unreasonable amount of vibration which is conducted to the surrounding framework. In addition, the cable between the lifting tray and the winch is susceptible to premature wear, thus limiting the maximum loads for the lifting mechanism. The engagement of the cable on the pulleys further adds to the noise problem of the winch. Another problem resides in the particular mounting of the lifting tray upon the track assembly which has occasionally led to binding and jamming during raising and lowering of the loaded tray. Also, the unprotected nature of the winch motor and the battery on the mounting plate exposes these elements to moisture, temperature and other environmental conditions which can affect their maintenance, operability and reliability.
For the above reason, the prior art design has not proved entirely effective and convenient to use. Accordingly, it has been found desirable to provide an improved ladder with a lifting tray that offers the following advantages over the prior art systems including a smoother and quieter lifting operation; a more efficient, reliable and better controlled drive system for the lifting tray; and, retrofit capability. Such a ladder and lifting tray should be easy to assemble and maintain, and more economical to produce.